This invention relates to the plasma arc cutting process for cutting metal sheets or plates into desired shapes. In this process plasma is formed by ionizing a gas in the extreme heat of an electric arc. Gas molecules absorb heat energy from the arc, dissociate into atoms, and then further break down into electrically-charged particles called ions. As described in U.S. Pat. No. 2,806,124, the "ionized" gas is forced through a very small nozzle and accelerated to form an intensely-hot, constricted arc that can melt metals. Water injection may be used to improve metal cut quality. As described in U.S. Pat. Nos. 3,567,898 and 3,641,308, water enters an annular chamber in the plasma arc nozzle below the plasma constriction point, and then passes out around the plasma jet. In so doing, it helps to further constrict the plasma jet, making it hotter and better defined. Upon striking the plate, the water removes excessive heat and inhibits slag adherence.
However the plasma arc cutting process generates considerable noise and vapor while in operation. The vapors are unpleasant and require a venting system, or used on a limited basis so that operating personnel are not adversely affected by the vapors.
One technique which has been used to materially reduce the noise and vapors emitting from the plasma arc is a water muffler. As described in U.S. Pat. No. 3,833,787, the water muffler includes a nozzle which fits on the plasma arc torch which directs a high flow of water in a shield around the plasma arc. The muffler water flow is initiated at the same time the plasma arc is started to muffle the noise generated in the jet mixing region at the exit of the nozzle and to prevent vapors from escaping from the torch and/or workpiece. The water muffler system is preferably used in connection with a water reservoir located below the workpiece to be cut, to prevent high intensity sound waves from escaping out of the bottom of the cut plate, U.S. Pat. No. 3,787,247.
The path that the torch follows is automatically controlled through known electrical scanning systems. This automatic control includes a control system for maintaining the torch at the proper height above the workpiece, as the torch cuts the workpiece into a desired shape.
In one control system the height of the plasma arc torch above the workpiece is controlled by a differential amplifier. A detector circuit indicates the distance between the plasma torch and the workpiece. This can be done for example, on the basis of the capacitance between a sensing electrode as one plate, the work piece as the other plate of the capacitor, and the air dielectric. The differential amplifier compares the output voltage of the detector circuit with a reference voltage circuit which includes a manually adjustable height control potentiometer. A difference between the detector voltage and the reference voltage causes the differential amplifier to generate an output of the correct polarity and magnitude to raise or lower the torch to a level at which the detector voltage is equal to the reference voltage.
When used with such a torch height control system, the water muffler produces so much water (as much as three-sixteenth inch) on the workpiece that the automatic height control of the plasma arc torch is disrupted, apparently because of the difference in dielectric properties of air and water. The water increases the capacitance and decreases the detector voltage. The resulting detector voltage applied to the differential amplifier is lower than normally would be the case with little or no water on the workpiece. This causes the differential amplifier to raise the plasma torch, as described in the preceeding paragraph, to a distance too high above the workpiece for efficient cutting of the workpiece by the plasma arc torch.
To correct this problem, the torch may be lowered by adjustment of a plasma torch height control potentiometer. However when the cutting operation is completed, the water muffler is automatically turned off. When the next workpiece to be cut is moved into the cutting fixture, there is little or no water from the water muffler system on the workpiece. If the potentiometer is adjusted to take into account the considerable water on the surface of the workpiece due to the water muffler and little or no water is present, the plasma torch will be driven into the workpiece, causing damage to the plasma torch.
The muffler water cannot be run continuously to maintain water on the workpiece without modification of the work area to provide an extensive drainage and/or recycle of a large volume of muffler water, which in many cases would be impractical.
Other material on the workpiece such as oil, grease and cleaning liquid may also cause changes in the detector voltage applied to the differential amplifier, and will thus raise or lower the torch to a level other than the desired cutting height.